Chapter 3 Summary & Mechanism Review

Alkenes primarily undergo electrophilic additions. Predict outcomes by knowing the intermediate type.

Core Reactions

• HX (hydrohalogenation): Markovnikov alkyl halides via carbocations; rearrangements possible. HBr + peroxides → anti-Markovnikov radical path.
• Hydration: Acid hydration (Markovnikov, possible rearrangements); oxymercuration–demercuration (Markovnikov, anti, no rearrangements); hydroboration–oxidation (anti-Markovnikov, syn).
• Halogenation / Halohydrin: X₂ → vicinal dihalides (anti via halonium). X₂/H₂O → halohydrin (OH to more substituted carbon, anti).
• Dihydroxylation: OsO₄ or cold KMnO₄ (syn diols); epoxide + acid open (anti diols).
• Hydrogenation: H₂/Pd (syn) → alkane.
• Oxidative cleavage: Ozonolysis → carbonyls (or acids with oxidative workup); hot KMnO₄ similar.
• Polymerization: Strong acid or radicals can initiate cationic/radical chains.

Mechanism Patterns

1. Carbocation path: HX, acid hydration. Watch Markovnikov and rearrangements.
2. Three-membered ring: Halonium/mercurinium; anti attack, defined regioselectivity.
3. Concerted: Hydroboration, catalytic hydrogenation; syn, no rearrangements.

Regio/Stereo Reminders

• Markovnikov vs anti-Markovnikov controlled by reagent set (e.g., HBr ± peroxides; hydration vs hydroboration).
• Anti products from halonium openings; syn from concerted additions.
• Racemization possible when planar carbocations form new stereocenters.

Quick Checklist

• Identify the likely intermediate (carbocation, halonium, concerted).
• Apply the corresponding regio/s stereo rule.
• Consider rearrangements only for carbocation pathways.